Conductor Ampacity Calculation – Part Six

In Article " Conductor Ampacity Calculation – Part Four "  I explained the rule that controlling and refining the Conductor Ampacity selection from NEC Tables.


Also, in Article " Conductor Ampacity Calculation – Part Five ", I explained Correction/ adjustment Factors Affecting Conductor Ampacity Tables as provided in 310.15(B)


Today, I will explain some special cases for conductor ampacity calculations as follows.


For more information and good following, please review the following articles: 

• Conductor Ampacity Calculation – Part One

• Conductor Ampacity Calculation – Part Two 

• Conductor Ampacity Calculation – Part Three

Special Cases for Conductor Ampacity Calculations

Some special cases for the application of adjustment factor of table 310.15(B)(3)(a) where is More Than Three Current-Carrying Conductors in a Raceway or Cable are as follows:

1. Bare or Covered Conductors, 
2. Neutral Conductor, 
3. Grounding or Bonding Conductor, 
4. Main feeders to Dwelling units, 
5. Conductors contained in sheet metal auxiliary gutter, 
6. Conductors contained in Nonmetallic Auxiliary Gutters, 
7. Conductors in Metal Wireways, 
8. Conductors for Remote Remote-Control, Signaling, And Power-limited Circuits, 
9. Conductors for Fire Alarm Systems, 
10. Conductors and Cables in Cable Trays. 

Case#1: Bare or Covered Conductors Where bare or covered conductors are installed with insulated conductors, the temperature rating of the bare or covered conductor shall be equal to the lowest temperature rating of the insulated conductors for the purpose of determining ampacity.

Case#2: Neutral Conductor This Case discuss considering the neutral conductor as a current carrying conductor or not. If it will be a current carrying conductor it will be counted when applying the Adjustment factor of table 310.15(B)(3)(a) where is More Than Three Current-Carrying Conductors in a Raceway or Cable. The below table summarize different situations of case#2.

Neutral is a current carrying conductor – apply table 310.15(B)(3)(a)	Neutral is not a current carrying conductor – don’t apply table 310.15(B)(3)(a)
In circuits having a 3-wire circuit from a 4-wire, 3-phase, 208Y/120V or 480Y/277V (see fig.1).	If Neutral carries only the unbalanced current from other conductors of the same circuit.
In balanced 4-wire, 3-phase, circuit that has nonlinear loads exceeds 50% of its loads (see fig.2).	In Balanced circuits like 3 wire, 1-phase, 120/240 V or 4-wire, 3-Phase, 208Y/120V or 480Y/277V (see fig.3).

Fig (1)

Fig (2)

Fig (3)

Definition: Nonlinear Load:  A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.

Example#1:

What is the minimum size THHN conductor required for the branch circuit conductors for a circuit of 120 volts, with a 10,000 watt load operating for 3.5 hours , located in a 65° C environment with (3) ungrounded conductors, (1) grounded conductor, and (1) grounding conductor in conduit, connected to 75° C terminations and devices ? Existing load is fluorescent lighting.



Solution:

Step#1: Get Current = watts / volts = 10,000/120 = 83.3 amps

Step#2: load operating for 3.5 hours, so it will be a continuous load

The THHN Conductor is sized for Continuous current loading = 83.3 X 125% = 104.125 Amps 

Step#3: application of Adjustment factors 

Since more than 50% 0f the load is nonlinear loads (fluorescent lighting), so the neutral will be a current carrying conductor – apply table 310.15(B)(3)(a)

From Table 310-15(b)(3)(a) and for (4-6 conductors), the adjustment factor = 80%

From Table 310-15(b)(2)(a) and for an ambient temperature 65° C and temperature rating of conductor 90° C , the ambient temperature correction factor = 0.65

Step#4: Get Wire corrected Ampacity

Corrected Ampacity = 104.125 / (0.8 x 0.65) = 200.24 amps

Step 4: Get suitable Wire size

From table 310.15(B)(16) , for THHN Conductors but for 75° C terminations, we will use 75° C Column.

You find the suitable conductor size is 4/0 AWG.

Case#3: Grounding or Bonding Conductor A grounding or bonding conductor shall not be considered current carrying conductor and not counted when applying the adjustment factor of  table 310.15(B)(3)(a) where is More Than Three Current-Carrying Conductors in a Raceway or Cable.

Case#4: Main feeders to Dwelling units 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. These conductors can be sized according to Table 310.15(B)(7) with condition that it carries 100 percent of the dwelling unit’s diversified load.

Important!!! the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit. (see fig.4) Fig (4) In fig.4, Left side, the reduced conductor size permitted is applicable only to the service-entrance conductors run to each apartment from the meters. In fig.4, Right side, the reduced conductor size permitted is also applicable to the feeder conductors run to each apartment from the service disconnecting means, because these feeders carry the entire load to each apartment and are considered to be the main power feeder to each apartment.

Notes to Case#4: The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded (neutral) conductor for dwelling units can be smaller than the ungrounded conductors, but in no case it is smaller than required to carry the maximum unbalanced load as determined in 220.22. in addition the grounded (neutral) conductor shall not be smaller than that required by 250.24(B). Table  310.15(B)(7) does not apply to 3-wire, 1-phase 208Y/120V systems, because the grounded (neutral) conductor in these systems carries neutral current even the load on the phases is balanced 310.15(B))4(6).

Example#2:

What size service/feeder conductors would be required if the total adjusted demand load for a dwelling unit equals 195 A and the maximum unbalanced load is 70A?



Solution: 

Step#1: ungrounded service/feeder conductors size 

From Table 310.15(B)(7) , and for adjusted demand load for a dwelling unit 195 A

The Feeder size will be 2/0 AWG rated 200 A. 

Step#2: the Grounded (neutral) conductor size 

250.24(B) requires the grounded (neutral) conductor to be sized no smaller than 4 AWG based on table 250.66

Since the maximum unbalanced load is 70A and from table 310.15(B)(16), 75° C column

The Grounded (neutral) conductor size will be 4 AWG, rated 85 A at 75° C.

Case#5: Conductors contained in sheet metal auxiliary gutter As per NEC 366.23(A), Where the number of current-carrying conductors contained in the sheet metal auxiliary gutter is 30 or less, the adjustment factors specified in Table 310.15(B)(3)(a) shall not apply.

Important!!! As per NEC 366.23(A): The current carried continuously in bare copper bars in sheet metal auxiliary gutters shall not exceed 1.55 amperes/mm2 (1000 amperes/in.2) of cross section of the conductor. For aluminum bars, the current carried continuously shall not exceed 1.09 amperes/ mm2 (700 amperes/in.2) of cross section of the conductor.

Case#6: Conductors contained in Nonmetallic Auxiliary Gutters As per NEC 366.23(B), The adjustment factors specified in 310.15(B)(3)(a) shall be applicable to the current carrying conductors in the nonmetallic auxiliary gutter.

Case#7: Conductors In Metal Wireways As per 376.22(B), the conductor ampacity requirements will be as follows: Where the quantity of current-carrying conductors does not exceed 30, the adjustment factors of 310.15(B)(3)(a) do not apply. Where the quantity of current-carrying conductors does exceed 30, the adjustment factors of 310.15(B)(3)(a) do apply. Neutral conductors are determined to be current-carrying conductors according to 310.15(B)(5). Signaling circuit conductors are not considered current carrying conductors. Conductors used only for motor-starting duty as limited by 376.22(B) are not considered current-carrying conductors.

Important!!! As per 376.22(A),  the number of conductors in metal wireways will be determined as follows: All conductors count in determining wireway fill. Total conductor fill must not exceed 20 percent of the wireway.

Case#8: Conductors For Remote Remote-Control, Signaling, And Power-limited Circuits As per 725.51, the conductor ampacity requirements will be as follows: (A) Where Class 1 circuit conductors are permitted in a raceway, The ampacity adjustment factors given in 310.15(B)(3)(a) shall apply only if such conductors carry continuous loads in excess of 10 percent of the ampacity of each conductor. (B) Where power-supply conductors and Class 1 circuit conductors are permitted in a raceway, The ampacity adjustment factors given in 310.15(B)(3)(a) shall apply as follows: To all conductors where the Class 1 circuit conductors carry continuous loads in excess of 10 percent of the ampacity of each conductor and where the total number of conductors is more than three To the power-supply conductors only, where the Class 1 circuit conductors do not carry continuous loads in excess of 10 percent of the ampacity of each conductor and where the number of power-supply conductors is more than three

Important!!! Class 1 circuits: The portion of the wiring system between the load side of the overcurrent device or power-limited supply and the connected equipment. Class 1 circuits shall be classified as follows: Class 1 power-limited circuits Class 1 remote-control and signaling circuits Class 1 Power-Limited Circuits: These circuits shall be supplied from a source that has a rated output of not more than 30 volts and 1000 volt-amperes. Class 1 Remote-Control and Signaling Circuits: These circuits shall not exceed 600 volts. The power output of the source shall not be required to be limited.

Case#9: Conductors for Fire Alarm Systems As per 760.51, the conductor ampacity requirements will be as follows: (A)- Where only non–power-limited fire alarm circuit and Class 1 circuit conductors are in a raceway, The ampacity adjustment factors given in 310.15(B)(3)(a) shall apply if such conductors carry continuous load in excess of 10 percent of the ampacity of each conductor. (B)- Where power-supply conductors and fire alarm circuit conductors are permitted in a raceway, The ampacity adjustment factors given in 310.15(B)(3)(a) shall apply as follows: To all conductors where the fire alarm circuit conductors carry continuous loads in excess of 10 percent of The ampacity of each conductor and where the total number of conductors is more than three. To the power-supply conductors only, where the fire alarm circuit conductors do not carry continuous loads in excess of 10 percent of the ampacity of each conductor and where the number of power-supply conductors is more than three. (C)- As per 760.130, Where only non–power-limited fire alarm circuit conductors are in a raceway, The ampacity adjustment factors given in 310.15(B)(3)(a) shall not apply.

In the next Article, I will explain the Last Special Cases for Conductor Ampacity Calculations: Conductors and Cables in Cable Trays. Please, keep following.

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